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Supplemental Coal Data (Unaudited) (Tables)
 12 Months Ended
Dec. 31, 2016
SUPPLEMENTAL COAL DATA [Abstract]  
Reserves Reported to Other Agencies [Table Text Block]
Supplemental Coal Data (unaudited)
 
 
Millions of Tons
 
 
For the Year Ended December 31,
 
 
2016
 
2015
 
2014
 
2013
 
2012
Proven and probable coal reserves at beginning of period
 
3,047

 
3,238

 
3,032

 
4,229

 
4,314

Purchased reserves
 
—

 
24

 
—

 
1

 
—

Reserves sold in place
 
(601
)
 
(43
)
 
(233
)
 
(1,199
)
 
(155
)
Production
 
(26
)
 
(29
)
 
(32
)
 
(55
)
 
(55
)
Revisions and other changes
 
(59
)
 
(143
)
 
471

 
56

 
125

Consolidated proven and probable coal reserves at end of period* (1)
 
2,361

 
3,047

 
3,238

 
3,032

 
4,229

 
 
 
 
 
 
 
 
 
 
 
Proportionate share of proven and probable coal reserves of unconsolidated equity affiliates (excluded from the table above)*
 
—

 
—

 
55

 
57

 
41

______________
* Proven and probable coal reserves are the equivalent of “demonstrated reserves” under the coal resource classification system of the U.S. Geological Survey. Generally, these reserves would be commercially mineable at year-end prices and cost levels, using current technology and mining practices.